Genetics: Polymorphisms, Epigenetics, and Something In Between

Figure 2

An illustration of a balance between heterochromatic sequences and heterochromatin components (e.g., proteins or RNAs). Repetitious heterochromatin-forming sequences (rectangles) are normally in balance with the proteins that bind them (circles), package them as heterochromatin, and thereby stabilize them (conditions in gray). Since these factors are used to regulate expression of euchromatic genes, the balance must accommodate “excess” factors for that purpose (denoted as circles apart from rectangles). If the expression or activity of proteins is reduced (a), repetitious sequence is exposed, destabilized, and lost through damage-repair, recombination, or extrachromosomal circle formation (b), until a new balance is established. Excess protein has gene regulatory consequence throughout the genome and presses to reestablish balance by altering expression level or activity (c) or perhaps through repeat expansion (d).